A great deal of energy is expended to calcine a calcium carbonate-containing material such as limestone to calcium oxide as is required, for example, in the production of lime and hydraulic cements. Most lime is produced in energy-intensive rotary or shaft kilns. The process is relatively simple in that particulate materials are fed into refractory lined furnaces, mixed with suitable fuels, and subjected to intense heat caused by the combustion of the fuel. Provisions are made to optimize exposure of the feeds to the thermal environment and the system is an open one to permit escape of the products of calcination; lime and carbon dioxide and traces of moisture. The heating or calcination of limestone into lime requires breaking an oxygen-to-carbon bond and liberating carbon dioxide. The calcination reaction is represented by the equilibrium equation: EQU CaCO.sub.3 .revreaction.CaO+CO.sub.2
In commercial practice the actual energy employed is many times greater than the theoretical bond strength due to heat losses, heat transfer properties, and the impetus required to remove the liberated carbon dioxide. Large amounts of energy are also required in fluidized bed calciners.
Large amounts of energy are used in such calcination to hasten calcination and promote the removal of CO.sub.2 from the reaction bed. Such usage, while not expensive when fuel was inexpensive, was wasteful and, in some cases, when excessive temperatures were used, often resulted in overburing or hard burning and caused the reaction of lime with impurities producing hard grains in the lime.
With escalation of energy costs due to increases in the price of fossil fuels such as oil and coal, many efforts have been undertaken to conserve energy and reduce costs in this energy intensive process. These have included changes in apparatus, the use of air purges, evacuation, or collection of the carbon dioxide, and the use of "dry feed" practices to eliminate the need for large amounts of energy to remove water from the calcined product. While effecting some reduction in fuel usage, they have not been entirely successful in cutting costs.
It is to be understood, of course, that the energy consumption in the production of lime by calcination of limestone is also applicable to the cement industry where the calcination of limestone represents a major portion of the energy-consuming process.